Add Polaris FastAPI backend: models/schemas, pricing+scoring+order router, importer, Celery tasks, API routers, seed script

backend/README.md

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+# Polaris Backend
+
+FastAPI + SQLAlchemy 2.0 + Pydantic v2 + Celery/Redis backend for the Polaris
+reseller platform. PostgreSQL schema is canonical in `/opt/polaris/db/schema.sql`
+(not managed by this service).
+
+## Run (CT host, no Docker)
+
+```sh
+cd /opt/polaris/backend
+python3 -m venv .venv && .venv/bin/pip install -r requirements.txt
+# .env at /opt/polaris/.env provides POSTGRES_PASSWORD (auto-loaded)
+.venv/bin/python scripts/seed.py                      # sample supplier + 27 products
+DATABASE_URL=postgresql+psycopg2://reseller:${POSTGRES_PASSWORD}@127.0.0.1:5432/reseller \
+  .venv/bin/uvicorn app.main:app --host 0.0.0.0 --port 8000
+```
+
+## Run (Docker, via OPS compose)
+
+The OPS agent adds `api` + `worker` services. Inside the Docker network:
+- `DATABASE_URL=postgresql+psycopg2://reseller:${POSTGRES_PASSWORD}@postgres:5432/reseller`
+- `REDIS_URL=redis://redis:6379/0`
+- worker command: `scripts/celery_worker.sh` (worker + beat; beat schedule in `app/tasks.py`)
+
+## Env vars
+
+| var | purpose |
+|---|---|
+| `DATABASE_URL` | SQLAlchemy URL (default: local 127.0.0.1:5432 w/ `POSTGRES_PASSWORD`) |
+| `REDIS_URL` | Celery broker (default `redis://redis:6379/0`) |
+| `SECRET_KEY` | JWT signing key (dev default — set in prod) |
+| `ADMIN_EMAIL` / `ADMIN_PASSWORD` | admin login (plain compare — TODO: hash `ADMIN_PASSWORD_HASH` with bcrypt/argon2) |
+
+## API
+
+All under `/api` (see CONTRACT.md for the full list). Interactive docs at `/docs`.
+
+Key notes:
+- Importer NEVER auto-publishes: new products enter as `status='imported'`;
+  `POST /api/products/{id}/publish` requires a calculated `retail_price`.
+- Pricing: active `price_rules` bands (cost → markup), floor at
+  cost+shipping+fees, `min_price` respected; `estimate_net_profit` =
+  retail − cost − shipping − fees.
+- Orders: `POST /api/orders` routes each item to the highest-scoring eligible
+  supplier (40% price / 20% inventory / 15% speed / 10% fulfillment / 10%
+  returns / 5% history) and records profit.
+- Analytics `conversion` is 0.0 until a traffic/visitor source exists.
+
+## Seed
+
+`scripts/seed.py` — idempotent. Creates the sample supplier, imports 27
+products (home/gadgets/office, $50–$500 retail), prices via the pricing engine,
+and publishes the valid ones.